The proposed research program aims to unravel the persisting mystery of hemispheric specialization and interhemispheric interaction in the human mind/brain. We will carry out coordinated experiments with normal subjects, hemisphere damaged patients, and split brain patients, using convergent behavioral and physiological methods. The program addresses general issues in cognitive neuroscience concerning modularity, intermodular communication, and control of parallel processing in independent modules. A core set of three lateralized behavioral experiments will investigate (1) hemispheric specialization and interdependence, (2) explicit and implicit interhemispheric transfer, and (3) interhemispheric control of parallel processing. Each experimental paradigm includes an intrahemispheric component for studying hemispheric independence, and an interhemispheric component for studying interhemispheric interaction and control. The same behavioral experiments will be administered to patients with (1) complete cerebral commissurotomy, (2) agenesis of the corpus callosum, and (3) partial callosotomy, both pre- and post-surgically. The first paradigm is dichotic listening to words and accents. It measures complementary left hemispheric specialization for linguistic phonetic analysis and right hemisphere specialization for social/pragmatic aspects of communication. The second paradigm is lexical decision of lateralized targets with distractors in the opposite visual hemifield. It measures independent word recognition, independent error monitoring in the two hemispheres, and implicit priming across the hemispheres. The third paradigm is perceptual matching of letters by shape or by name within and between the hemispheres. It measures the ability of the hemispheres to process information in parallel in complex tasks. Together, the three experimental paradigms developed in this proposal provide a compact but comprehensive and exquisitely sensitive battery of tests of interhemispheric relations. This battery can then be used for studying individual or group differences in interhemispheric relations in normal and pathological populations, such as acquired aphasia, congenital dyslexia and schizophrenia. Results promise better understanding and possible control and reversal of pathology due to abnormal cerebral activation or impaired interhemispheric communication.